U.S. patent number 6,677,752 [Application Number 09/716,621] was granted by the patent office on 2004-01-13 for close-in shielding system for magnetic medical treatment instruments.
This patent grant is currently assigned to Stereotaxis, Inc.. Invention is credited to Francis M. Creighton, IV, Gareth T. Munger, Peter R. Werp.
United States Patent |
6,677,752 |
Creighton, IV , et
al. |
January 13, 2004 |
Close-in shielding system for magnetic medical treatment
instruments
Abstract
A shield for controlling a magnetic field emanating from a
magnetic in a magnetic medical treatment instrument has the form of
a circular shaped band with opposite open ends and a center axis
extending perpendicularly between the open ends. The magnet in the
magnetic medical treatment instrument moves about an operating
table having a surface upon which a patient being operated on by a
physician reposes. The circular shaped shield surrounds the magnet
and is spaced away from the magnet in a manner to allow the
physician to enter the interior of the band and operate on the
patient. The band has opposite first and second parallel planar
portions. The top surface of the operating table and the first and
second planar portions of the band are parallel to each other. The
operating table has a length and a width, and the length of the
operating table is parallel to the center axis of the band.
Inventors: |
Creighton, IV; Francis M. (St.
Louis, MO), Munger; Gareth T. (Richmond Heights, MO),
Werp; Peter R. (St. Louis, MO) |
Assignee: |
Stereotaxis, Inc. (St. Louis,
MO)
|
Family
ID: |
29780736 |
Appl.
No.: |
09/716,621 |
Filed: |
November 20, 2000 |
Current U.S.
Class: |
324/318;
324/319 |
Current CPC
Class: |
G01R
33/421 (20130101) |
Current International
Class: |
G01V
3/00 (20060101); G01V 003/00 () |
Field of
Search: |
;324/318,319,320,322,300,312,314 ;335/296,301 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
4646046 |
February 1987 |
Vavrek et al. |
4651099 |
March 1987 |
Vinegar et al. |
5001448 |
March 1991 |
Srivastava et al. |
|
Primary Examiner: Arana; Louis
Attorney, Agent or Firm: Harness, Dickey & Pierce,
P.L.C.
Claims
What is claimed is:
1. A shield for shielding the magnetic field produced by a magnetic
medical device which includes a magnet, the shield surrounding the
magnetic medical device and having opposite ceiling and floor
sections and opposite left and right sections extending between the
ceiling and floor section, the ceiling, floor, left and right
sections defining an operating space, the floor section being at a
different distance from the magnet than the ceiling section.
2. The shield of claim 1 wherein the sections are formed from
layers of aluminum and steel.
3. The shield of claim 2 wherein the layers of aluminum and steel
are bonded together with adhesive to form a rigid laminate
member.
4. The shield of claim 1 wherein: the ceiling and floor sections of
each define a plane, the left and right sections each have a top
edge abutting the ceiling section and a bottom edge abutting the
floor section, the top and bottom edges of each of the left and
right section are contained entirely within the plane of both the
ceiling and floor sections.
5. The shield of claim 1 wherein: the left and right sections have
a top edge abutting the ceiling section and a bottom edge abutting
the floor section, the top and bottom edges of the left and right
sections form smooth transitions in the operating space between the
left and right sections and the ceiling and floor sections.
6. The shield of claim 1 wherein the ceiling section extend outward
beyond the left and right side sections to form overhang.
7. The shield of claim 1 wherein: each of the left and right
sections have a bottom edge adjacent the floor section and a floor
angle plate attached to each of the bottom edges of the left and
right sections, the floor angle plate joins each of the left and
right sections to the floor section, the floor angle plate provides
a smooth transition from each of the left and right sections to the
floor section.
8. The shield of claim 7 wherein: the floor section includes a
floor box with four walls extending upward from the floor section
on a side perimeter edge of the floor section, each of the four
walls of the floor box surrounds the angle plate and entirely
contains the floor angle plate within the floor box.
9. The shield of claim 1 wherein the operating space has the form
of a tube having opposite open ends spaced apart by the left and
right side sections and a center axis between the open ends.
10. A shield for shielding a magnetic field generated by a magnetic
medical device which includes a magnet, the shield comprising: a
tube having cylindrically shaped sides spaced apart by opposite
planar ceiling and floor panels, the floor panel being at a
different distance from the magnet than the ceiling panel, the tube
having opposite open ends separated by the sides and a center axis
extending between the open ends, the tube having an interior
surface defining an operating volume in which the magnetic medical
treatment instrument is placed, the operating volume having
sufficient space to allow a physician to enter and exit the
operating volume through one of the open ends of the tube and
operate the magnetic medical device.
11. The shield of claim 10 wherein the magnetic medical treatment
instrument includes an operating table having a length and a width,
and the length of the table is parallel to the center axis.
12. The shield of claim 10 wherein the cylindrically shaped sides
smoothly transition to the planar ceiling and floor panels on the
interior of the tube.
13. The shield of claim 10 wherein: each of the left and right
sections have a bottom edge adjacent the floor section and a floor
angle plate attached to each of the bottom edges of the left and
right sections, the floor angle plate joins each of the left and
right sections to the floor section, the floor angle plate provides
a smooth transition from each of the left and right sections to the
floor section.
14. The shield of claim 13 wherein: the floor section includes a
floor box with four walls extending upward from the floor section
on a side perimeter edge of the floor section, each of the four
walls of the floor box surrounds the angle plate and entirely
contains the floor angle plate within the floor box.
15. The shield of claim 10 wherein the planar ceiling and panel
extends outward from the operating volume and overhangs the
cylindrically shaped sides.
16. The shield of claim 10 wherein the magnetic field emanating
from the magnetic medical treatment instrument is guided by the
cylindrically shaped sides through the open ends of the tube.
17. A shield for controlling the magnetic field emanating from a
magnetic medical device wherein the magnetic medical device has a
magnet that produces the magnetic field, the magnet moves about an
operating table having a top surface upon which a patient reposes
while being operated on by a physician, the shield comprising: a
circular shaped band having opposite open ends and a center axis
extending perpendicularly between the open ends, the band being
circumsuperjacent the magnet in a manner to allow the physician to
enter an interior of the band and operate on the patient, the band
having opposite first and second parallel planar portions, the top
surface of the operating table and the first and second planar
portions being parallel to each other, the operating table having a
length and a width and the length being parallel to the center axis
of the band, and the first planar position being closer to the
magnet than the second planar position.
18. The shield of claim 17 wherein the first and second planar
portions extend outward and away from the band.
19. The shield of claim 17 wherein the first and second planar
portions have opposing faces that are smoothly formed in the
interior of the band.
20. A method of designing a shield for a magnetic medical device
which includes a source magnet, the shield comprised of a floor
plate and a ceiling plate, the method comprising: a) balancing the
size of the two plates according to their solid angle subtended at
the source magnet; b) balancing the thickness of the two plates
according to their solid angle subtended at the source magnet, in
combination with appropriate materials; c) choosing the size of the
plates according to their effectiveness in controlling the flux
lines beyond them, as attested by the pattern of the flux leakage
around them; and d) using an iterative design procedure employing a
finite element analysis program.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to magnetic shielding and in particular to
passive magnetic shielding used to contain the fringe magnetic
field emanating from a source magnet contained in a magnetic
medical treatment instrument.
2. Description of the Related Art
Magnetic medical treatment instruments have long been used in the
medical field as a tool in performing non-invasive medical
procedures, stereotaxic mapping, and magnetic resonance imaging
(MRI). The versatility for providing care with these types of
instruments has increased with the advent of more powerful
computers to assist the physician in controlling the magnet and
processing the data developed by the treatment instrument. However,
the fields and gradients created by the source magnets used by
these treatment instruments are strong and usually require
extensive shielding at many health care facilities.
In the environment of a health care facility, a magnetic field may
cause interference with the proper operation of health care
monitoring equipment and other electronic health aids. In areas
where magnetic medical treatment instruments are used, advisory
signs are generally posted to warn about the potential dangers of
entering the area. Rooms housing magnetic medical treatment
instruments or treatment rooms, have restricted access when the
instrument is being used so that patients and other personnel using
magnetically sensitive electronic equipment do not inadvertently
enter the treatment room are become adversely effected from the
operation of the instrument. Since space is at a premium in health
care facilities, magnetic medical treatment instruments may be
placed in rooms that are adjacent other areas where health care
monitoring equipment and other electronic health aids are being
used. To limit the areas in the health care facility having
restricted access, the treatment room must be properly shielded.
Some institutions and agencies impose limits as low as 5 Gauss on
the field strength of magnetic fields outside the treatment
room.
Conventional shielding systems in treatment rooms at health care
facilities are rather elaborate and generally fixed in the
structural portions and foundations of the treatment rooms. For
example, the walls, the ceiling, and the floor of the treatment
room may be lined with iron-based or other magnetic permeable
materials in order to deflect and control the magnetic field
generated by the source magnet. This shielding is extremely heavy,
and may require additional structural support. Thus installation
can be difficult, time consuming and expensive.
Often, conventional room shielding must be customized on the site
once the magnetic medical equipment is installed to ensure proper
attenuation of the magnetic field from the source magnet. These
unexpected problems sometimes pose logistical problems for
technicians during the initial set-up of the treatment instrument.
Highly permeably magnetic materials are generally not easily
machined and assembled on the job site. Congruent and matching
shapes must be created between adjoining sections of shield to
reduce the possibility of fringing the magnetic field.
A primary reason that room magnetic shielding is so heavy and
complete is that it is used for MRI machines. These are generally
hug cylindrical coils which are kept at a strong current
continuously for months and years. Because of the size, the field
wall (unless the new, compensating coils are used) is relatively
strong. In addition, the size and direction of the field is
constant, so that a strongly magnetized shielding, tending in part
toward saturation at the wall, will take on a somewhat permanent
magnetization of its own, with thermal variations and mechanical
vibrations present to realign the magnetic domains more completely.
It is observed when MRI machines are turned off that the wall
shielding is quite highly magnetized. Unless such shielding is
thick and of more expensive very low carbon steel, such a
magnetization can be sever, even while maintaining safe levels
outside the room, and will be insufficient to maintain safe levels
outside after a period of use.
Therefore, in the Magnetic Stereotaxis System, or other strong
magnetic field sources which take random directions in a procedure
room, it is possible to judiciously design a system with much
lighter, thinner, and less encompassing shielding.
Therefore, what is needed is a magnetic shield that guides and
shapes the magnetic field emanating from the source magnet of a
magnetic medical treatment instrument in such a manner as to
attenuate the field in a relatively short distance away from the
magnet. The shield would be arranged in close proximity to the
magnet to contain the magnetic field around the magnet but still
allow proper operation of the magnetic medical treatment
instrument. By attenuating the field in a short distance away from
the source magnet, rooms adjacent to the treatment room may have
unrestricted access. The shield would allow other magnetic
sensitive equipment found in rooms adjacent to the treatment room
to be operated without interference. The magnetic shield would be
standard equipment for a particular medical treatment instrument
and obviate the requirement for custom placement and configuration
of special shielding in the treatment room. The magnetic shield
would be smaller and be of less weight so that the structural
requirements of the treatment room may be reduced. The magnetic
shield would have a lower cost than conventional methods of
shielding the entire ceiling, floor, and walls of the treatment
room.
SUMMARY OF THE INVENTION
According to the principals of the present invention, a magnetic
shield is arranged in close proximity to the source magnet of a
magnetic medical treatment instrument and forms an operating space
within the volume defined by the shield. The magnetic shield shapes
and channels the fringe magnetic field from the source magnetic to
allow the magnetic medical treatment instrument to be operated in
an environment where other magnetically sensitive electronic
equipment may be used.
In one embodiment of the current invention, a magnetic shield is
provided around a magnet medical device that generates magnetic
fields to shape and channel the field so as to contain the magnetic
field in the immediate vicinity of the magnet. Thus, the shield
prevents the magnetic field from radiating from the treatment room
into surrounding rooms which may have other sensitive electronic
equipment that may be disturbed or disrupted by the magnetic field
generated by the magnetic medical treatment instrument. The
magnetic shield has a ceiling and opposite floor section, and left
and right side sections extending between the ceiling and floor
sections. The ceiling and floor sections and left and right
sections of the shield define the operating space. The left and
right side sections are curved members having concave surfaces
facing toward each other that give the operating space a generally
cylindrical shape. The magnetic reduced device is positional within
the operating space.
According to another embodiment of the invention, the shield has
the form of a tube with cylindrically shaped sides spaced apart by
parallel, planar ceiling and floor sections. The tube has opposite
open ends separated by the sides, and a center axis extending
between the open ends. The tube has an interior defining an
operating space in which the magnetic medical treatment instrument
is placed. The operating space has sufficient area to allow a
physician to enter and exit the operating space to operate on a
patient with the magnetic medical treatment instrument.
According to another embodiment of the invention, a shield is used
for controlling the magnetic field emanating from a magnetic
medical device. The shield has the form of a circular shaped band
having opposite open ends and a center axis extending
perpendicularly between the open ends. The band is positioned to
surround the magnet at distance from the magnet to allow the
physician to enter the interior of the band and operate on the
patient. The band has first and opposite, second parallel planar
portions. The top surface of the operating table and first and
second planar portions are parallel to each other.
In this arrangement the magnetic field emanating from the magnet
used in the magnetic medical device may be shaped and contained
with the immediate area of the magnet. By guiding the magnetic
field in this way, the field may be directed out the open ends of
the band into the surrounding room in selected directions. Because
magnetic field falls off quickly with distance from the source, the
magnetic field channeled out through the open ends of the shield
may be dissipated in selected directions without the use of
additional shielding in the walls of the treatment room. By
providing the shield in close proximity to the source magnet, the
magnetic field emanating from a magnetic medical device may be
contained within the boundaries of the treatment room so that the
operation of the magnetic medical device does not disrupt other
electronic and other sensitive medical monitoring and treatment
equipment in adjacent rooms. This also allows the construction of
smaller treatment rooms. Additionally, by constructing a shield in
close proximity to the source magnet, a smaller shield may be
provided with a reduced weight and lower cost than room
shielding.
In some cases a room is large enough, or a magnet source has small
enough projected field, that shields on the ceiling and floor will
suffice. In those cases, it is desirable to minimize their sizes
and weights. This can optimally be done with appropriate
consideration of the relative sizes and distances of these flat
shields to that of the source magnet. Usually the floor plate is
closer to the source magnet than is the ceiling plate. Therefore
its design will need to incorporate greater thickness to reduce
saturation, while it may be somewhat smaller in extent because it
subtends a larger solid angle at the closer distance to the magnet.
In essence, these plate provide regions which do not surround the
source magnet, but which instead provide effective return paths for
the flux at room boundaries that are closer than the side
walls.
With the principle just stated, a finite element analysis (FEA)
program can be used to provide specific design by trial and error.
In this method, a first trial set of floor and ceiling plates is
located in the FEA volume and a worst case magnetic source field
applied. The resulting fields in regions beyond the plates and
within the plates are calculated. From these the degree of
saturation or lack of it in the plates is noted, and sizes and
thickness of the plates adjusted accordingly, depending on the
material used. A difficulty in this procedure occurs because the
source field is so much stronger than the tolerable field outside
the shields that sufficient resolution in space and field strength
cannot be obtained in practical times with readily available
computers. At this juncture in the procedure it is important to
assess the field leakage around the plates along with the fields in
the plates. If the leakage bulges sharply and significantly at the
plate edges, then the plate is not overly saturated, but is too
small, at least in one dimension. If the field drops across the
plate, but is still too large beyond it, and there are not sharp
bulges at the edge, then the plate is too thin, or of a material
which saturates too easily, or is of too low permeability.
BRIEF DESCRIPTION OF THE DRAWINGS
Further objects and features of the invention are revealed in the
following detailed description of the preferred embodiment of the
invention and in the drawings wherein:
FIG. 1 is a perspective view of a magnetic medical treatment
instrument of the present invention with a close-in magnetic shield
system in place around the instrument;
FIG. 2 is a perspective view of the magnetic shield of FIG. 1 with
a cut-away view of a connection between a left section of the
shield and a floor section of the shield;
FIG. 3 is a cross-sectional view of a joint in a section of the
magnetic shield of FIG. 1;
FIG. 4 is a partial, side cross-sectional view of a typical portion
of the magnetic shield used to form a floor section of the magnetic
shield of FIG. 1;
FIG. 5 is a partial, side cross-sectional view of a typical portion
of the magnetic shield used to form ceiling, left, and right
sections of the magnetic shield of FIG. 1.
FIG. 6 is an exploded, perspective view of the magnetic shield of
FIG. 1;
FIG. 7 is a partial, side view of the connection between the
ceiling section of the shield and the left side section of the
magnetic shield of FIG. 1;
FIG. 8 is a partial, side cross-sectional view of the connection
between the floor section of the shield and left side section of
the magnetic shield of FIG. 1;
FIG. 9 is a side view of the magnetic shield of FIG. 1 showing the
magnetic field distribution from the side of the shield; and
FIG. 10 is a front view of the magnetic shield of FIG. 1 showing
the magnetic field distribution from the front of the shield.
Corresponding reference characters indicate corresponding parts
throughout the several views of the drawings.
DETAILED DESCRIPTION OF THE INVENTION
FIGS. 1 and 2 show a general construction of a magnetic shield
constructed according to the principles of the present invention.
The magnetic shield, generally indicated at reference number 10,
includes a ceiling section 12 and an opposite, floor section 14
spaced apart by a left and right side sections 16, 18. The ceiling
section 12 and floor section 14 are preferably rectangular in shape
and form parallel planes spaced apart from one another by the side
sections 16, 18. The left and right side sections 16, 18 are curved
members with concave surfaces facing each other that give the
shield 10 a generally cylindrical appearance with opposite open
ends 20, 22. Together, the ceiling and floor sections 12, 14, and
the left and right side sections 16, 18 define the boundaries of an
operating space 24. The operating space 24 has a center axis
extending between the open ends 20, 22 that is aligned parallel
with the axial direction of the cylindrically shaped shield 10 and
the left and right side sections 16, 18 of the shield. Each of the
ceiling and floor sections 12, 14 extend outward beyond the open
ends 20, 22 of the operating space beyond the side edges of the
left and right side sections 16, 18 in the direction of the center
axis. Preferably, the ceiling section 12 extends outward beyond the
left and right side sections 16, 18 in a direction perpendicular to
the center axis to form an overhang 26 with each of the left and
right side sections 16, 18.
A magnetic medical device 28 is positioned within the operating
space 24. The magnetic medical device 28 includes a patient table
30 with a longitudinal axis that is aligned with the center axis of
the operating space 24, and a source magnet 32. As shown in FIGS. 1
and 2, the floor 34 of the treatment room is equipped with a track
36 to allow the source magnet 32 to be positioned with respect to
the patient table 30. In an alternative construction, the treatment
instrument may have a patient table capable of moving into the
operating space. The track 36 for the treatment instrument 28 is
aligned parallel with the center axis of the operating space 24.
The track 36 stops in a position where the source magnet 32 is
fully encompassed by the left and right side sections 16, 18 when
the source magnet 32 is properly positioned relative to the patient
table 30.
Inside the operating space 24, the longitudinal axis of the patient
table 30 is preferably positioned parallel with the center axis of
the shield 10 as shown in FIGS. 1 and 2. A top surface 31 of the
patient table 30 upon which the patient reposes during the
procedure preferably forms a plane that is parallel with the
ceiling and floor sections 12, 14 of the shield. Preferably, the
patient table 30 is positioned such that the portion of the
patient's body to be treated using the source magnet 32 is
positioned within the travel range of the source magnet 32. A
portion of the patient table 30 may extend through one of the open
ends of the shield 10 so as to provide a more compact arrangement
of the shield 10 around the patient table and the properly
positioned source magnet 32.
The ceiling and floor sections 14, 16 may be formed from smaller
modules 38 to reduce the overall weight and cost of manufacturing
the sections of shield 10. Preferably, the ceiling and floor
sections 14, 16 are made from three modules 38 and each of the left
and right side sections 16, 18 are made from a single module 38.
Each module 38 is preferably an industry standard size of
4'.times.8' flat stock material. As shown in FIG. 3, each module to
be joined to form a section has a joint edge 40 with steps 42. The
steps 42 allow the modules 38 to be joined with an overlap that
forms a smooth interlocking surface between adjacent modules 38 of
the sections. The outer most corners of the modules may have
fastener holes to secure adjacent modules. The left and right side
sections 16, 18 of the shield are preferably rolled to form the
desired radius of curvature to generate the needed volume in the
operating space 24.
As shown in FIGS. 3, 4 and 5, the modules 38 are preferably made
from layers of carbon steel 44 and aluminum 46. Experimentally, it
has been found that a laminate of AISI 1008 steel and aluminum (any
grade) provide adequate materials for the shield. The carbon steel
44 is a magnetically permeable material and the aluminum 46
decreases the overall weight of the module 38. Preferably, each
layer of the carbon steel 44 is 1/32" thick and each layer of the
aluminum 46 is 1/16" thick. The floor section 14 of the magnetic
shield 10 is comprised of four layers of carbon steel 44 interposed
among three layers of aluminum 46, as shown in FIG. 4. The ceiling
section 12, and left and right side sections 16, 18 of the magnetic
shield have a similar construction except that the laminate is
comprised of three alternating layers of carbon steel interposed
among two layers of aluminum, as shown in FIG. 5. To bond the
carbon steel 44 to the aluminum 46, an adhesive 48 is used.
Preferably, the adhesive 48 is sprayed on and has a thickness of no
more than 0.010" to promote adequate field conduction.
The ceiling section 12 of the magnetic shield is preferably held in
position above the operating space 24 by attaching it to a
structure of the room in which the magnetic medical treatment
instrument 28 is to be used. The ceiling section 12 is suspended
from the structural members that comprise the ceiling structure
between adjacent floors in the building. The ceiling section 12 is
rectangular in shape and extends beyond the operating space 24 to
overhang the patient table 30 at one open end 20 of the operating
space 24 and to overhang the magnetic medical treatment instrument
28 as it travels along its tracks 36 adjacent the opposite open end
22 of the operating space 24.
Spaced away and parallel to the ceiling section 12 is the floor
section 14. In a similar arrangement with the ceiling section 12,
the floor section 14 preferably extends beyond the operating space
24 in the directions of the open ends 20, 22 of the operating space
24 along the center axis of the operating space 24. The floor
section 14 may project the same distance as the ceiling section 12
beyond the open ends 20, 22 of the operating space 24 in the same
direction as the center axis of the operating space 24. As shown in
FIG. 2, the floor 34 of the treatment room is preferably formed
with a rectangular recess 50 having a length and width equal to the
floor section 14 of the magnetic shield 10 so that the recess 50
may receive the floor section 14 therein. The depth of the recess
50 is sized for the thickness of the floor section 14 and to
accommodate the height of the tracks 36 upon which the source
magnet or patient table slides. The side perimeter edges of the
floor shield 14 operably connect to the left and right side
sections 16, 18 of the shield 10 and do not extend beyond the left
and right sections 16, 18.
The left and right side sections 16, 18 of the shield are concave
members that are shaped to increase the volume of the operating
space 24. The radius of curvature generally is proportional to the
shape of the magnet field emanating from the source magnet 32 so as
to contain the flux emanating from the magnet 32. The curvature is
also arranged so that a smooth transition may be provided between
the left and right side sections 16, 18 and each of the ceiling and
floor sections 12, 14 of the shield. The left and right side
sections 16, 18 are positioned so that when the magnet 32 is
properly positioned with respect to the patient table 30, the left
and right side sections 16, 18 fully encompass the source magnet
32. The left and right side sections 16, 18 are also spaced away
from the source magnet 32 to allow the physician to enter the
operating space 24 and operate on a patient on the patient table
30. The left and right side sections 16, 18 are spaced sufficiently
away from the patient table 30 to allow the physician to enter and
exit the operating space 24 and access the control panels and other
instrumentation that are used to control the treatment instrument
28. Since the ceiling and floor sections 12, 14 are permanently
affixed to the structures of the treatment room, the left and right
side sections 16, 18 do not bear the weight of the ceiling section
12. The left and right side sections 16, 18 of the magnetic shield
10 are constructed to support each section's 16, 18 respective own
weight while the curvature provides ample room in the operating
space 24.
The left and right side sections 16, 18 of the shield act as a flux
connector between the ceiling and floor sections 12, 14 of the
shield. In order to guide the field between each of the side
sections 16, 18 and the floor and ceiling sections 12, 14 without
causing the field to fringe, a smooth transition between the
sections is needed. To provide the smooth transition between the
left and right side shields 16, 18 and the ceiling and floor
sections 12, 14 of the magnetic shield, ceiling and floor angle
plates 52, 54 are provided.
As shown in FIG. 6, two ceiling angle plates 52 are provided to
join the left and right side sections to the ceiling section 12,
and two floor angle plates 54 are provided to join the left and
right side sections 16, 18 to the floor section 14 of the shield.
Preferably, the angle plates 52, 54 have the same width as the left
and right side sections 16, 18. The ceiling angle plates 52 have a
side engagement portion 56 that attaches a top edge 58 of the each
of the left and right side sections 16, 18 of the shield, and a
ceiling engagement portion 60 that is obliquely angled to the side
engagement portion 56. The top edge 58 of each of the left and
right side sections 16, 18 and the side engagement portion 56 of
the ceiling angle plates 52 have a series of matching fastening
holes 62 that permit attachment of each of the side sections 16, 18
to the respective ceiling angle plate 52 to the ceiling section 12
of the shield. As shown in FIG. 7, the oblique angle at which the
ceiling engagement portion 60 is formed with the side engagement
portion 56 provides a smooth transition between the left and right
side sections 16, 18 and the ceiling section 12. The angle plate 52
may have a thickness that allows mechanical fasteners 66 to be
countersunk into the ceiling angle plate 52 so as to prevent a
fringing field to be developed from sharp protrusions that may
extend beyond the interior surfaces of the shield.
To join the left and right side sections 16, 18 of the shield to
the floor section 14, the floor angle plates 54 are provided. The
floor angle plates 54 have a similar construction to the ceiling
angle plates 52 in that the floor angle plates 54 have a side
engagement portion 68 that attaches to the left and right side
sections 16, 18 of the magnetic shield and a floor shield
engagement portion 70 which engages and attaches to the floor
section 14. The side engagement portion 68 of the floor angle plate
54 and the bottom portion of each of the left and right side
sections 16, 18 have a series of matching holes 72. In this way,
each left and right side section 16, 18 may be attached to the
respective floor angle plate 54. Preferably, the mechanical
fasteners 66 are used to join the left and right side sections 16,
18 to the floor angle plate 54. The side engagement portion 68 is
obliquely angled to the floor engagement portion 70. The floor
engagement portion 70 also has a series of holes through it along
its width 74 for attaching the floor angle plate 54 to the floor
section 14.
On the side perimeter edges of the floor section 14 in the area
where the left and right side sections 16, 18 are joined to the
floor section 14, a floor box 76 is provided. As shown in FIG. 6,
each floor box 76 is rectangular in shape with four walls extending
outward from a bottom wall 78 to form a rectangular box with an
open top. The floor box 76 has side walls 80 that are slightly
wider than the width of the left and right side sections 16, 18 of
the shield, and end walls 82 that are wider than the combined size
of the side engagement portion 68 and the floor engagement portion
70 of the floor angle plate 54. As shown in FIG. 8, the interior
volume of the floor box 76 is sized to receive the floor angle
plate 54 and the bottom portion of the respective left and right
side section 16, 18. The bottom wall 78 of the floor box 76 is
positioned on top of the floor section 14 to expose the open top.
The side perimeter edge of the floor section 14 is substantially
even with the outboard side wall 80 of the floor box 76. The floor
box 76 is anchored to the floor 34 of the treatment room through a
floor box anchor 84. The floor box anchor 84 also partially secures
the floor section 14 to the floor 34 of the room.
FIGS. 2 and 8 show the typical arrangement of the floor box 76 and
floor section 14. The walls 78, 82 of the floor box 76 extend
upward from the bottom wall 78 of the floor box 76 to enclose the
floor angle plate 54 and provide a bounded volume in which the left
and right side sections 16, 18 of the shield may be joined to the
floor section 14. The walls 78, 82 of the floor block 76 are spaced
away from the surfaces of the floor angle plate 54 to provide an
installation technician access to the mechanical fasteners 66 that
attach the left and right side sections 16, 18 of the magnetic
shield to the side engagement portion 68 of the floor angle plate
54 and access to the mechanical fasteners 66 that attach the floor
engagement portion 70 of the angle plate to the floor section
14.
Generally, a portion of the magnetic medical treatment instrument
28 slides on a track 36 that is positioned in the floor 34 of the
treatment room. To ensure that the magnetic flux emanating from the
source magnet 32 of the magnetic medical instrument 28 is properly
controlled and shaped, the floor section 14 of the shield is
positioned under the tracks 36, as shown in FIG. 8. However, to
support the weight of the sliding portions of the treatment
instrument 28, the tracks 36 are positioned on a layer of concrete
86 poured on top of the floor section 14 of shield. When the
concrete is poured over the floor section 14 of the shield and
trenches 88 for the tracks 36 are formed, the floor boxes 76
provide a mold around the fasteners 66 and floor angle plate 54 to
provide access to the floor angle plate 54 and the fasteners 66.
The floor box 76 acts a flux connector to direct the magnetic field
from the left and right side sections 16, 18 to the floor 14 where
the concrete layer 86 in the floor 34 might otherwise impede the
travel of the field. Preferably, the floor blocks 76 and floor
angle plates 54 are made from carbon steel or other highly magnetic
permeable material to allow the magnetic field to be conducted from
the ceiling section 12 to the floor section 14 of the shield.
FIGS. 9 and 10 provide a visual representation of the attenuation
of the magnetic field using the shield 10 of the present invention.
In operation, the shield 10 attenuates a magnetic field having a
field strength of 0.1 Tesla to less than 5 Gauss at a distance of
approximately ten feet from the top surface of the operating table
in each vertical direction. The 5 Gauss line is indicated at
reference numeral 90. Similarly, the shield 10 attenuates the field
to less than 5 Gauss at a distance of ten feet from the centerline
of the patient table in each horizontal direction. The shield
directs the magnetic field out through the open ends 20, 22 of the
operating space 24 where air acts to attenuate the magnetic field.
As shown in FIGS. 9 and 10, the field is also attenuated to less
than 5 Gauss at a distance of approximately ten feet in both
directions through the open ends 20, 22 of the shield 10.
Thus, with the magnetic shield 10 of the present invention, the
magnetic medical treatment instrument 28 may be positioned in any
room in the hospital and an adjacent room may have unrestricted
use. Although the patient table 30 extends upward from the floor 36
of the treatment room a height of roughly four feet, the distance
between the rooms on floors below and above the treatment room is
such that the 5 Gauss line 90 does not generally extend into these
areas. Thus, rooms on floors below and above the treatment room may
also be released for unrestricted use given the shield and source
magnet described previously.
The ceiling angle plate 52 and the floor angle plate 54 provide
smooth transitions to guide and shape the flux between the ceiling
and floor sections 12, 14. Since the shield 10 is arranged in close
proximity to source magnet 32, the size and resultant weight of the
shield 10 may be reduced. This reduces the material and cost of the
shield 10. Moreover, because the shield is relatively thin and has
a reduced weight, the shield may be easy installed and retrofitted
into existing treatment rooms. Although the magnetic shield 10 of
the present invention is formed using alternating layers of carbon
steel 44 and aluminum 46, other highly magnetic permeable materials
such as 80 Ni metal may be used to form the shield 10. Carbon steel
44 has been chosen because of its relative cost effectiveness, its
each of machining and manufacturing, its bonding capabilities with
other metals, and its ease of forming into required shapes.
As various changes could be made in the above construction with
departing from the scope of the invention, it is intended that all
matter contain the above description and shown in the accompanying
drawings shall be interpreted as illustrative and not in any
limiting sense. The invention therefore shall be solely limited by
the scope of the claims set forth below.
* * * * *